The detection and tracking of targets is typically accomplished by a variety of radar systems that analyze the time difference of arrival, Doppler shift, and various other changes in the reflected energy, to determine the location and movement of targets. Phased array antenna systems employ a plurality of individual antenna elements or subarrays of antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front or cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array travels in a selected direction. The differences in phase or timing among the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives. Such processing as described above is well known to those of ordinary skill in the art.
A pulse based radar system scans a field of view and emits timed pulses of energy. Such radar systems, including, for example, CTA type radar systems, can require both short range and long range target detection and tracking. Long range (e.g. on the order of 60 kilometers (Km) or more) detection performance requires relatively long pulse repetition intervals (PRI). A narrow beam is typically required for long range target detection and tracking.
For CTA radars especially and for full 360° coverage the single array is often rotated at high angular rates to obtain the look opportunities needed for target detection, track, and localization for estimation of launch or impact points. Due to high target vertical velocities, rotation rate, and elevation beam widths, the number of look opportunities is limited.
Usually, the problem of short range detection of a 360° (degree) scanning radar has been solved by rotating a single array phase at a rapid angular rate. One issue with such an approach is that for short range targets, there is no option for increasing coverage other than beam spoiling. This is tends to be less efficient than other methods such as increasing rotation rate, which can create mechanical problems.
A conventional radar array contains a plurality of radiating elements configured to define an array aperture for generating a narrow beam for long range detection and track performance. The longer PRI reduces the probability of detecting high vertical velocity, shorter range targets (e.g. targets within about 15 Km). In order to alleviate this problem, systems may utilize separate short range (SR) and long range (LR) pulses in an attempt to cover all target ranges. However, even with SR pulses, significant limitations exist in conventional radar systems processing and implementation.
For example, short range detection and localization performance of conventional radar systems is typically not limited by target signal-to-noise ratio (SNR), but rather by the number of look opportunities of the target by the radar. This number is limited by such factors as high target vertical velocities, elevation beamwidth, and target revisit rate. More specifically, short range target detection and localization is usually not a function of SNR, because such short range targets typically have SNRs well in excess of typical threshold detection levels. However, a problem lies with the number of look opportunities with which to detect, track and localize a target with sufficient accuracy to evaluate a projectile launch or impact point. A radar system utilizing a narrow beam long range pulse for detecting and tracking targets may operate quite effectively for long range objects; however, such a system may be inadequate to track short range objects having high target vertical velocities, which require much greater processing and response time, but which does not require such narrow beam(s). Alternative techniques for detecting and tracking both long range and short range targets within a single radar system are desired.
The present invention relies in part on recognition of the aforementioned problems, and in providing a solution for enhancing a radar's target coverage without significantly impacting its long range or short range performance. The present invention operates to electrically and mechanically separate a full aperture radar into multiple apertures.